1. Field of the Invention
This invention relates to an overhead projector, and more particularly to an overhead projector capable of projecting a picture image of substantially fixed magnitude with a constant brightness onto a screen regardless of the size of a transparent positive placed on a stage glass thereof.
2. Description of the Prior Art
A typical overhead projector of prior art comprises a projector body having a light source system disposed in a housing so as to face upward, a condenser lens disposed above the light source system, and a stage glass closing a top opening of the housing, whereby a transparent positive is placed on the stage glass and luminous flux coming through the transparent positive is converged by the aforesaid condenser lens at a point outside the housing and then diverged by a projecting lens, for projecting a picture image of the transparent positive onto a screen of a certain dimension by means of a projecting reflective mirror.
With such an overhead projector of the prior art, in order to project a picture image which is clear throughout the entire screen of the certain dimension, the size of the stage glass is made substantially similar to that of the transparent positive and the light source system in the housing is provided with a sufficient luminous power and a sufficient luminous flux for projecting the transparent positive. Thus, the overhead projector of the prior art has shortcomings in that it is impossible to project a transparent positive having a size larger than the dimension for which the projector is designed and that, when a transparent positive which is smaller than the aforesaid dimension is used, a relatively small picture image proportionate to the size of the transparent positive is projected on the screen, so that the excessive glaring at the periphery of the screen around the projected small picture image disturbs the appreciation of picture image and makes it very difficult to effect detailed checking and study of a complicated picture image.